The invention relates to a method for starting the modulation of a frequency converter. More specifically, the invention relates to a method for starting the pulse width modulation of a frequency converter when the frequency converter is equipped with a common-mode output filter.
The output voltage of a pulse-width modulated (PWM) inverter may cause problems in motor drives. Harmonics at the switching frequency and its multiples give rise to additional losses and acoustic noise in the motor, and pulse reflections expose the motor insulations to additional voltage stresses.
These problems can be reduced by adding a sinusoidal LC filter having the cut-off frequency well below the switching frequency to the output of the inverter. Furthermore, to avoid capacitive leakage currents and bearing currents, the filter can be designed to reduce the common-mode voltage.
FIG. 1 shows an output filter that reduces both common-mode (CM) and differential-mode (DM) voltages. The similar topology was originally disclosed in H. Akagi, H. Hasegawa, and T. Doumoto, “Design and performance of a passive EMI filter for use with a voltage-source PWM inverter having sinusoidal output voltage and zero common-mode voltage,” IEEE Trans. Power Electron., vol. 19, no. 4, pp. 1069-1076, July 2004.
An LC filter, consisting of a three-phase inductor Lf and three capacitors Cf, attenuates high frequencies of the differential-mode inverter output voltage. A CM inductor Lc, capacitor Cc and resistor Rc form a common-mode filter, which provides a route for the CM current and reduces the CM voltage at the motor terminals.
However, the common-mode filtering presents a problem. The filter may cause severe oscillation when the modulation is started. Due to the abrupt start of a pulsating CM voltage and zero initial values of the CM voltage and current in the filter, the CM inductor Lc may get saturated in the start. The saturated CM inductor causes high current peaks, which may trigger the overcurrent protection. The problem has been reported in N. Hanigovszki, “EMC output filters for adjustable speed drives,” Ph. D. dissertation, Inst. Energy Techn., Aalborg Univ., Aalborg, Denmark, March 2005 for a slightly different filter topology.
FIG. 2(a) shows a modulation start of a 2.2-kW induction motor drive (400 V, 5.0 A, 50 Hz) equipped with the output filter shown in FIG. 1. The highest current peak exceeds 30 A, and the CM inductor becomes saturated. In the worst cases, the oscillation continued for several seconds.
One solution for the starting problem was disclosed in the above Hanigovszki reference. This solution is based on additional hardware, which comprises a dynamic damping circuit consisting of an additional winding of the common-mode inductor, a full-bridge rectifier, a filtering capacitor and a resistive load. The damping circuit solves the starting problem, but it may affect the overall filter performance. Furthermore, the additional hardware makes the damping circuit less attractive with respect to physical size of the hardware and also to expenses.